How MRI works
Let’s just start off by saying that an MRI or “magnetic resonance imaging” is an awesome piece of technology (http://orthopedics.about.com/cs/sportsmedicine/a/mri.htm). You might have been to the doctor before and saw an MRI machine or you might have had to have an MRI done on you before. To the normal person it might look like a big cylinder shaped donut with a table that slides in and out of it. There is so much more going on inside it, and hopefully in this article we can touch on some of the basics of how an MRI works so that you can be more educated next time you see one or hear the name. On July 3, 1977 the first MRI was taken. It was a huge event in the world of modern medicine. Although the image was quite ugly compared to today’s standards, it was a big step towards what we have today. Dr. Raymond Damadian, Dr. Larry Minkoff, and Dr. Michael Goldsmith worked for seven years to get to this day in history. Because it was such a hard process and many thought that it couldn’t be done, they named their first machine the “Indomitable.” This machine is now in the Smithsonian Institute. By the year 1982 there were still only a handful of MRI scanners in the U.S. Today there are millions of them operating, and the technology has come quite a long ways in only 30 years, compared to X-ray which has been around for over 100 years. Before going into an MRI scan, the patient is checked and rechecked to make sure there is nothing magnetic in or on them. Metal fragments in the eye could cause eye damage or blindness. If a patient has a pacemaker, it could be liable to malfunction and throw off a persons heart beat, which could be fatal. Aneurysm clips in the brain could move and tear the artery that they were in place to repair. Most orthopedic implants are ok because they are firmly embedded in bone. Even metal staples that have been in a patient for about six weeks are considered ok, because they are held in place by scar tissue. Although an MRI does not cause any biological hazard to humans, pregnant women are usually referred to other types of imaging. This is because not much is known about the biological effects on a developing fetus. The MRI scan takes place when the person is slid inside the bore of the machine on a table. The bore is the long horizontal tube that the person is slid into. The unit can be as big as 7’ tall x 7’ wide x 10’ long, although they are now getting smaller with new technology. An old MRI machine can weigh 17000 lbs, and a newer one might weigh around 9700 lbs. The great cut in weight is because of smaller magnets being able to produce the same amount of magnetic intensity as the bigger and older magnets. This will be referred to later on in this report. The person can be slid all the way into the bore or just partially, depending on what part of their body needs to be scanned. Once the body part to be scanned is in the exact center of the magnetic field, the scan can begin. An MRI scanner can basically pick out a very small point inside a person’s body and produce an image of it. The point could be as small as a cube that is a half millimeter on each side. It also can do a scan of the whole body. MRI’s are so valuable because they can basically show the inside of a person’s body without using a process like X-ray, which has potential to harm the patient. Also they can show a patients blood flowing in any part of the body, and not show any of the tissue around it. This can be accomplished without the need of a contrast injection, unlike in vascular radiology. You might be wondering why nothing magnetic can go in or even be around the MRI machine while it is in use. This is because the most important part in an MRI system is its magnet. The magnet is measured in units of measure known as the tesla and the gauss (1 tesla = 10,000 gauss). Today’s MRI use magnets that range from 0.5-2.0 tesla or 5,000-20,000 gauss. This is a very powerful magnet when compared to the earth’s magnetic field of only 0.5 gauss. Any metal object that gets too close to the MRI unit can be pulled in and stuck to the magnet. This is very dangerous when a patient is getting a scan, so extreme caution has to be taken to not allow any metal objects around the MRI. There are three basic types of magnets used in MRI systems. : 1. Resistive magnets- consist of many coils of wire wrapped around a cylinder through which an electric current is passed. This causes a magnetic field to be generated, and if the electricity is turned off the magnetic field goes away. These magnets are fairly cheap to construct but the electricity to run them is very expensive because of the resistance in the wire to the flow of electrons. : 2. Permanent magnet- It is just a big magnet whose magnetic field is always on. There is no electricity or coils involved. Because of this it costs nothing to maintain the magnetic field, but the disadvantage is the weight of the magnet. For example to get a magnet to produce 0.4 teslas, the magnet would weigh many, many tons. This is definitely undesirable. : 3. Superconducting magnet- This is by far the most commonly used. It is somewhat the same as a resistive magnet, with its coils wrapped around a cylinder with current flowing through the coils causing a magnetic field. But in this magnet the wire coils are continuously dipped in a vacuum of liquid helium, which makes them to freeze at 452 degrees below zero. This causes the resistance in the wire to drop to zero, therefore using a lot less electricity in order to maintain its magnetic field. This magnet is very expensive, but it can produce 0.5-2.0 tesla fields, which make for a higher quality image. (www.howstuffworks.com) Also inside an MRI machine are three gradient magnets. They are much lower in strength than the main magnet, ranging from 180 – 270 gauss. Their main purpose is to allow the magnetic field to be directed to a specific point in the body, as will be discussed later on. (http://orthopedics.about.com/cs/sportsmedicine/a/mri.htm) Now that you know about the big magnet in an MRI system, let’s talk about the atomic structure inside a person’s body and how the magnets interact with it. The human body has billions of atoms in it, where the nucleus of the atom spins around on its axis. Although there are many types of atoms in the human body, for MRI purposes we are only interested in hydrogen atoms. This is because its nucleus has a single proton in it, and when placed in a magnetic field, it has a strong tendency to line up with the direction of the magnetic field. As mentioned earlier, the patient lies down in the center of the tube, and this is where the magnetic field is. This means that if the patient is lying on their back, the hydrogen protons will line up and point either towards the patients head or their feet. When they are all aligned like this, the majority of them cancel each other out, but there are still enough protons to make a quality image. Now that these billions of protons are all lined up, let’s see what happens next. (www.howstuffworks.com) The MRI machine applies a radio frequency (RF) pulse to all these hydrogen atoms that are in alignment. The system directs the pulse towards the part of the body to be examined. The protons in this part of the body absorb this energy, which makes them spin in a different direction at a particular frequency. At about the same time that this is taking place, the gradient magnets start to do their job. When they are turned on and off in a specific pattern, they can change the magnetic field in the exact area that the doctor wants a picture of. This is also referred to as slices, because an MRI can slice any part of the body in any direction, which means that the patient doesn’t have to move in order for the machine to get an image. This is a huge advantage over other imaging methods. Now when this radio frequency is turned off, the hydrogen protons return back to their natural alignment in the magnetic field like they were before the radio frequency was turned on. During this time they also release excess stored energy that they have, because they are moving from a high energy state to a low energy state. This exchange of energy is called resonance, thus the name Magnetic Resonance Imaging. When they do this, they act as a small radio transmitter and send off a signal that the scanner in the coil picks up and sends to the computer system. This signal is then converted from mathematical data, into a picture for all to see. If you have ever looked at an MRI image, there are white and dark areas on it. This is because different tissues in the body emit different energy levels, and the computer recognizes their distinct pulses. The difference in energy is determined by the atoms realignment time to their normal alignment. Dense tissue has less hydrogen atoms in it, thus taking longer to align than soft tissue. When the computer receives these different energy levels, it creates an image. An example of dense tissue would be bone, and it is shown as white on an image. An example of soft tissue would be a vein, and it is shown as black on an image. (www.howstuffworks.com), (www.ehow.com) There are so many advantages to an MRI that it wouldn’t make sense for me to list them, as it could take a number of pages. Because of this I will just list a few of the disadvantages related with getting an MRI done. · As mentioned before, people with pacemakers or certain types of metal in them cannot have an MRI done. · If a person is claustrophobic, it might be a bad experience for them if they even make it to the point of getting one done. · The machine makes a lot of noise. This is due to the gradient magnets opposing the main magnetic field. · A patient has to hold very still for possibly 20 – 90 minutes. This could be a problem for some people. · MRI systems are very expensive, therefore it costs a lot of money to have one done on you. If you don’t have insurance, it could end up costing a lot of money out of your pocket. Hopefully through reading this, you have gained some insight and knowledge into how an MRI works. There is a lot going on in one of them “donut looking” machines, and it is interesting to have a basic understanding of what is happening. Now the next time you see an MRI machine or hear the name, you will understand what it stands for and how it works.